The fuel asemblies of water-cooled nuclear reactors are generally constituted by a bundle (or cluster) of so-called fuel rods which consist of tubes containing pellets of nuclear fuel and are arranged parallel to each other and in such a way as to form a regular lattice (or network) in the assembly's plane of cross-section.
The fuel rods are held in place transversely by cross-bracing grids which are distributed along the length of the assembly, each of the grids reproducing the lattice in which the fuel rods are located.
The grids are constituted by plates cut into a ribbon and shaped to constitute bearing places for the fuel rods, such as bosses formed by embossing the metal of the plate. The plates are then welded together to form the grid lattice.
In the case of pressurized water nuclear reactors, the cross-bracing grids which hold the fuel rods in place are made in conventional manner from a precipitation-hardened nickel base alloy. This material possesses good mechanical properties which remain at a satisfactory level during the use of the fuel assemblies in the core of the reactor.
With the objective of improving the performance of nuclear fuel assemblies, the tendency nowadays is to replace the nickel alloy cross-bracing grids by cross-bracing grids consisting of plates of a zirconium-base alloy, the neutron capture cross-section of which is very much smaller. In fact, the fuel assemblies presently manufactured and used in pressurized water nuclear reactors incorporate, for the most part, grids made of a zirconium alloy, designated by the name zircaloy 4, which, in addition to zirconium, contains essentially tin, iron and chromium.
The manufacturing sequence of the cross-bracing grid, namely, the cutting, shaping and assembling by welding of the plates, is carried out on the material while in its final metallurgical state, i.e., on an alloy which has been heat treated to obtain the desired mechanical and chemical characteristics.
In order to improve the performance of the nuclear fuel, the present-day tendency is to increase the requirements for the as far as the characteristics are concerned, of the structural elements such as, for example, the cross-bracing grids.
In the main, these requirements refer to obtaining low resistance to the flow of the cooling water as it passes through the assembly, increased mechanical strength, and minimal alteration in the properties of the grid in the operating environment, namely, in the core of the nuclear reactor.
Obtaining low resistance to flow assumes the use of thin plates and complex geometry. The material which constitutes the plates of the grid should, in particular, be highly amenable to stamping.
The material should also be resistant to oxidation and to hydrogenization, and should undergo only very small dimensional changes in the core of the operating reactor.
Niobium-containing zirconium alloys are knonw, the properties of which, in particular their resistance to corrosion and to hydrogenization, can be adjusted by controlling their metallurgical state.
In certain metallurgical states, the said alloys can easily be cold-formed.
Finally, in whatever metallurgical state, the alloys containing at least 2% niobium have a mechanical resistance higher or much higher than that of Zircaloy 4 and a growth under irradiation which is weaker or much weaker.
However, to date, the said alloys have not been developed for components of fuel assemblies in pressurized water reactors (PWR), their resistance to generalized corrosion being considered as insufficiently compared to the corresponding resistance of zircaloy 4 at the operating temperature of a PWR (300.degree.-330.degree. C.). On the contrary, the resistance to corrosion of zirconium base alloys containing niobium at the operating temperatures of boiling water reactors in better than the corresponding resistance of zircaloy 4. Thus, these alloys are commonly used in boiling water reactors.
When it is desired to improve the performance of the grids and their useful life the utilization of these alloys is attractive for they have a slower kinetic of hydrogenization than zircaloy 4 in the operating environment of a PWR, as far as the other required properties can be optimized, i.e.:
the capacity for forming the semi-finished product (sheet metal),
the corrosion rate which should be as low as possible.
Further, the mechanical resistance and the growth under irradiation can also be optimized, thus increasing the advantages of the niobium containing alloys over zircaloy.
In German Patent Application 3 730 168, it is recommended to use zirconium base alloys containing niobium and tin for manufacturing components for nuclear reactor fuel assemblies. The components such as cross-bracing grids consist of plates which are cold rolled, cut and formed after a rapid quench from a high temperature (generally a water-quench). The components are tempered after being assembled by welding.